Itâ€™s been more than a month since the last time I did a detailed technical update, so for those who are interested here are some more of the gory details on our propulsion development work.

750-2B Testing and Vehicle Integration
At the end of my last post, we had just started firing the 750-2B engine. The -2B was the second rev 750, with a larger throat and longer L*, which had been modified with a bell nozzle. As mentioned, in addition to the bell nozzle, there were several other changes made.

These changes included cutting away a lot of material from the outside of the chamber near the throat and replacing it with an aluminum saddle, modifying the cooling groove geometry down near the nozzle end, expanding the cooling grooves in many areas where I thought we had excess cooling to try and cut down on the coolant channel pressure drop. After the series of test, it became apparent that while most of the changes had worked perfectly, I had gotten a little too aggressive on freeing-up pressure drop. While it wasnâ€™t clear that the engine couldnâ€™t steady-state, the thermal margin was much smaller than I wanted, and going for a steady state firing risked damaging the engine.

In the end, we decided that I didnâ€™t want to risk damaging our only flight engine before we had had the opportunity to fly. Our logic was that while weâ€™ve made four different regen cooled engine designs work so far as a company, we hadnâ€™t yet proven to ourselves that we could make an integrated vehicle fly. So, we decided to punt on trying to steady-state the -2B engine, and instead focus our efforts on flying. We had managed to run the engine for 15 seconds without any sign of damage, so we set a maximum flight duration of 9 seconds until we had the time to put the engine back on the test stand to make sure we could go longer, or until one of our other engines in the pipe had been steady-stated.